The semiconductor integrated circuit (IC) industry has experienced rapid growth. Technological advances in IC materials and design have produced generations of ICs where each generation has smaller and more complex circuits than the previous generation. However, these advances have increased the complexity of processing and manufacturing ICs and, for these advances to be realized, similar developments in IC processing and manufacturing are needed. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component that can be created using a fabrication process) has decreased.
During the fabrication of an IC, optical critical dimension (OCD) measurements may be made. OCD measurements may involve projecting a light beam to the wafer and performing the measurement based on the reflected light. For example, in a dual damascene process, after the metal trenches are formed and are being polished (for example in a chemical-mechanical-polishing process), it may be desirable to monitor the thickness of the trench being polished. This may be done to ensure that the metal trenches are not over-polished or under-polished. To accurately monitor the trench thickness, a light beam is projected to the layer in which the metal trenches are formed, and its reflection is measured. However, the accuracy of such measurements requires that the projected light does not penetrate to layers below the metal trenches. Otherwise, the reflected light may carry noise from the under-layers, thereby degrading OCD measurement accuracy.
As the scaling down process continues, it is increasingly more difficult to prevent the light from over penetration and/or to block the noise from the under layers. Therefore, while existing methods and structures of performing OCD measurement have been generally adequate for their intended purposes, they have not been entirely satisfactory in every aspect.